1. Field of the Invention
The present invention relates to a composition for forming a fluorescent film on a surface of the screen of a variety of display units and to a method of forming a fluorescent film for a display by using the composition above.
2. Description of Prior Art
A graphic fluorescent display tube, serving as a display unit, has a fluorescent film composed of a multiplicity of pixels to selectively cause the pixels to emit light by using collision of electrons so as to display an arbitrary graphic image. The fluorescent surface of the graphic fluorecent display tube is composed of, for example, fluorescent material ZnO:Zn when the graphic display tube is arranged to display a monochrome image. When the graphic display tube is arranged to display a full color image, a fluorescent film must be formed which is composed of a multiplicity of pixels by using fluorescent materials capable of emitting green (G), red (R) and blue (B) light. The fluorescent materials for use in the full color graphic display tube are, for example, ZnS:Cu, Al, which is a fluorescent material capable of emitting green (G) light, Y.sub.2 O.sub.2 S:Eu, which is a fluorescent material capable of emitting red (R) light and ZnS:Ag,Al which is a fluorescent material capable of emitting blue (B) light.
As a method of forming the fluorescent film into a predetermined pattern on the surface of a substrate or the like by using the above-mentioned fluorescent materials, a screen printing method, an electrodeposition method, a photolithography method and a slurry method have been known. However, the electrodeposition method is not a preferred method when a predetermined pattern must be formed by using the three types of fluorescent materials for green (G), red (R) and blue (B) light, as is required for the above-mentioned full color graphic fluorescent display tube, because undesired fluorescent materials are physically allowed to adhere. For example, a color CRT employs the slurry method to precisely pattern the three types of fluorescent materials for emitting green (G), red (R) and blue (B) light.
The slurry method has the step of mixing ammonium dichromate (ADC) as a photosensitive material with PVA (polyvinyl alcohol) water soluble resin so that photosensitive resin solution is prepared. Then, fluorescent particles are mixed with the solution so that slurry solution for use as a composition for forming a fluorescent film for a display is obtained. The side on which the fluorescent film will be formed, that is, a glass substrate having a surface on which the fluorescent film will be formed, is uniformly coated with the slurry solution, followed by drying the wet surface. Then, the glass substrate is irradiated with ultraviolet rays through a mask having a predetermined mask, and then water development is performed so that the portion exposed to the ultraviolet rays is retained. The foregoing process is repeated for each of the green, red and blue fluorescent materials. Then, the glass substrate is baked in an oxidizing atmosphere to decompose PVA and ADC with heat so as to evaporate and remove the same. Thus, the fluorescent film is formed.
If ADC is used as the photosensitive material in the slurry method, chrome oxide (CrO) is left on the surface of the fluorescent film after the baking process has been performed because ADC contains Cr. Cr is known to serve as a component for killing light emission even if Cr is contained in the fluorescent material in a small quantity. In a case of the CRT in which light emission is caused to take place in the inside portion of the fluorescent material by using high-speed electron beams, the killer effect can be prevented. However, the luminous efficiency of a fluorescent material of a type which emits light with relatively low speed electron beams (for example, acceleration voltage: 0.1 kV to 2 kV) sometimes deteriorates by 50% or more because of the killer effect above.
As another method of forming a fluorescent film capable of preventing retention of the killer in the fluorescent film, a PVA-SbQ method has been known. In this method, water soluble photosensitive resin is employed as the photosensitive material which has PVA as the main chain thereof and styryl pyridium group (abbreviated as a "SbQ group") as a side chain thereof to serve as the photosensitive group. The photosensitive material above is used similarly as is used in the slurry method in which ADC is used as the photosensitive material. PVA-SbQ is expressed by the following formula: ##STR1## wherein R.sup.1 is an alkyl group or aralkyl group, X.sup.- is a negative ion, m is 0 or 1, n is an integer 1 to 6, and ##STR2## is a quaternary aromatic heterocyclic group containing nitrogen.
Since the PVA-SbQ method does not use heavy metal, such as Cr. which is used in ADC, the light emission characteristic of the fluorescent material is not adversely effected. However, research and development performed by the inventors of the present invention proved that the SbQ group has intense ionicity similarly to ADC and thus, a fluorescent material having low tolerance to acid, for example, ZnGa.sub.2 O.sub.4 :Mn fluorescent material, can be damaged by ions of the SbQ group.
A fact has been found that use of a sulfide type fluorescent material for use in a general television display unit as a color fluorescent material for FED (Field Emission Display) results in the sulfide being scattered and the emitter being contaminated. In this case, a problem of reliability arises in that, for example, the lifetime of the FED deteriorates.
Accordingly a non-sulfide fluorescent material must be used as the fluorescent material for the FED in place of the sulfide fluorescent material. As a non-sulfide type fluorescent material capable of emitting green light, it might be considered to employ ZnGa.sub.2 O.sub.4 :Mn which is an oxide fluorescent material. However, when ZnGa.sub.2 O.sub.4 :Mn generally having poor tolerance to acid is mixed and dispersed in slurry solution containing the SbQ having the external salt structure, the surfaces of fluorescent material particles are denatured and, therefore, the luminous efficiency deteriorates. This phenomenon can be considered to be attributable to the inonicity of the SbQ group.